Return certain fields with .populate() from Mongoose

Question

I'm getting returned a JSON value from MongoDB after I run my query. The problem is I do not want to return all the JSON associated with my return, I tried searching the docs and didn't find a proper way to do this. I was wondering what if it is at possible, and if so what is the proper way of doing such. Example: In the DB

{     user: "RMS",     OS: "GNU/HURD",     bearded: "yes",     philosophy: {         software: "FOSS",         cryptology: "Necessary"     },     email: {         responds: "Yes",         address: "[email protected]"     },     facebook: {} }  {     user: "zuckerburg",     os: "OSX",     bearded: "no",     philosophy: {         software: "OSS",         cryptology: "Optional"     },     email: {},     facebook: {         responds: "Sometimes",         address: "https://www.facebook.com/zuck?fref=ts"     } }  

What would be the proper way of returning a field if it exists for a user, but if it doesn't return another field. For the example above I would want to return the [email][address] field for RMS and the [facebook][address] field for Zuckerburg. This is what I have tried to find if a field is null, but it doesn't appear to be working.

 .populate('user' , `email.address`)   .exec(function (err, subscription){      var key;     var f;     for(key in subscription){       if(subscription[key].facebook != null  ){           console.log("user has fb");       }     }   } 

Answer 1

I'm not completely clear on what you mean by "returning a field", but you can use a lean() query so that you can freely modify the output, then populate both fields and post-process the result to only keep the field you want:

.lean().populate('user', 'email.address facebook.address')   .exec(function (err, subscription){      if (subscription.user.email.address) {         delete subscription.user.facebook;     } else {         delete subscription.user.email;     }   });